Shells gleam more inside (4)
I believe the answer is:
ammo
'shells' is the definition.
(I've seen this before)
'gleam more inside' is the wordplay.
'inside' indicates the answer is hidden within the clue.
'AMMO' is hidden within 'gleam more'.
(Other definitions for ammo that I've seen before include "explosives, perhaps" , "rounds briefly" , "Bullets (sl.)" , "Bullets, shells, etc. (abbr.)" , "Bullets, etc. (colloq.)" .)